The present invention relates to an improved electrophotographic copying apparatus.
In an electrophotographic copier employing semi-moist development, a liquid toner substance is applied to a photoconductive drum to develop an electrostatic image of an original document formed thereon. A bias electrode is provided closely adjacent to the drum at the developing unit to which is applied a bias voltage of the same polarity as the electrostatic image on the drum. The potential of the bias voltage is selected to be slightly higher than the potential of white or background areas of the electrostatic image on the drum to prevent toner particles from being attracted to the background areas. This prevents the edge effect and ensures that the background areas will reproduce white.
However, it is difficult to select the proper bias voltage due to the great difference in original documents which must be reproduced. The proper bias voltage is a function of the density, contrast, image size and ratio of dark to light image areas of the document. Several types of systems have been proposed which automatically sense the average potential of the electrostatic image and adjust the bias voltage accordingly, thereby relieving the apparatus operator of this rather difficult task.
In one type of such system, the potential of the electrostatic image is sensed by means of an appropriately placed electrode and applied to an operational amplifier, which produces the bias voltage at its output. In another system the bias voltage is electrostatically induced on the bias electrode by the electrostatic image on the drum. However, even where various manual adjustments are provided to these systems they fail to provide copies of usable quality from certain kinds of documents.
Copies of photographs are often unsatisfactory since the dark areas appear washed out. Reproductions of diazo copies, which have low contrast, are often unreadable since the dark areas cannot be distinguished from the background areas. Copies of original documents having colored backgrounds are often unreadable since both the dark areas and the background reproduce with almost the same density. Whereas original documents have large dark areas, copies thereof often appear washed out and have low resolution. The latter problem is caused by an erroneously high bias voltage applied to the bias electrode.